We hypothesize that many, or most, children who have developed acute and/ or chronic graft vs. host disease (GVHD) post allogeneic bone marrow transplant (BMT) may have decreased bone mineral density that may place them at increased risk for fractures or avascular necrosis. Treatment with supplemental calcium and vitamin D may improve the bone mineral density, but there is no evidence that this therapy will improve BMD in these patients. The aims of this study are: 1) to determine the prevalence of decreased BMD in chidlren with active or inactive graft vs. host disease; 2) to correlate the BMD with duration of steroid and cyclosporine use (which is correlated with duration of GVHD), adjusting for other determinants of BMD, such as obesity, dietary intake of calcium and vitamin D, pubertal stage and physical activity; 3) to measure and compare the changes in bone mineralization over time in children with decreased bone density randomized to treatment wiht either calcium/vitamin D, or placebo. Thus far, all patients have tolerated the DEXA scan without problems. We have had excellence compliance with the diet history. Parents have not reported any trouble with the administration of the study medication. Parents of the younger children crushed the medication and mixed it with water. Five (5) more children will be entered in the next month. One child will need chloral hydrate for the DEXA scan.